The present invention relates to a method for producing a novel colored resin composition which has superior pigment dispersibility and impact resistance and which can be utilized as shaped articles, sheets and films made by injection molding or extrusion molding, the composition comprising a thermoplastic resin composition to which pigment is incorporated.
As coloring agents for filler-containing colored thermoplastic resin compositions, there are generally used unprocessed pigments or processed pigments in which metallic soaps such as zinc stearate and magnesium stearate or dispersion mediums such as wax and fatty acid are blended with unprocessed pigments Usually, pigments processed or not and fillers are added together to thermoplastic resins and the mixture is melt kneaded by extruders, Banbury mixers, kneaders or the like, thereby to improve properties such as heat resistance, rigidity, flame retardance, dimensional stability, printability, coatability and adhesion.
However, in the case of unprocessed pigments, high-performance kneading machines such as twin-screw extruders and Banbury mixers are required and besides color development is unstable In the case of simultaneous blending of processed pigments, fillers and thermoplastic resins, dispersion medium for the pigment are adsorbed to the fillers at the melt kneading step and no good dispersion of pigments is attained even by high-performance kneading machines such as twin-screw extruders and Banbury mixers. Such poor dispersion of pigments results in reduction of color density, uneven coloration and deterioration of properties and value of products is extremely lowered.
A conventional method for production of a filler-containing colored thermoplastic resin composition comprises simultaneously and uniformly mixing all of thermoplastic resins or thermoplastic resin compositions, fillers and pigments, melt kneading the mixture by melt kneading apparatuses such as extruders, Banbury mixers or kneaders and pelletizing. However, in the case where pigments are dry colored or master powder, this method has problems that the fillers are adsorbed to dispersion medium for the pigments at the melt kneading step and pigment agglomerate is produced, thereby reduction of color density, deterioration of gloss and decrease of impact strength are brought about owing to poor pigment dispersion.
An approach to solve the problem is made by carrying out simultaneous melt kneading in the same manner as above using a pigment master batch comprising dry colored or master powder and a thermoplastic resin used as a carrier resin, thereby to pelletize the master batch. According to this method, an improvement of pigment dispersibility is recognized, but the products are insufficient in heat resistance and impact resistance in the field of industrial parts which require high level in these properties. Furthermore, there are economical problems in that use of pigment master batch brings about increase in cost.